Nicotinamide phosphoribosyltransferase (Nampt) is a rate-limiting enzyme for synthesis of nicotinamide adenine dinucleotide in mammalian cells. Recent studies show that Nampt also works as a cytokine that exerts insulin-mimetic effects by binding to the insulin receptor, induces B-cell differentiation, inhibits neutrophil apoptosis, and affects immune and inflammatory functions. This review is focused on current knowledge regarding the structure and function of Nampt and its roles in carcinogenesis.